Field of the Invention
This invention relates to a system for establishing communications between a PLC (programmable logic controller) and other devices/appliances, and more particularly, to a communication system which can detect and display an abnormal portion in case of occurrence of abnormality in a communication route.
Description of Related Art
A programmable logic controller (hereinafter referred to as, “PLC” or “programmable controller”) is operated to establish communications with other processing devices. According to a Patent Document 1, the PLC judges that a communication abnormality has occurred when a data sent from one station to another was unable to be received for a predetermined time period and after a timer reached a predetermined time-up limit time. Further, according to another Patent Document 2, whether the data communication was successful or not, is confirmed based on the frequency of retrial processing by displaying the number of times of retrial processing.